33 years ago on her first day of work at a hospital, my friend’s mother inherited, in her words, an “antique doorstop and/or paperweight …we think it is petrified wood”. It is fist-sized, shiny, and much heavier than it appears. It is stumpy and rhombohedral-ish, with many semi-parallel lines along the sides and curving bands along one face. It kind of looks like petrified wood…but it is not. Far from it.
Petrified wood results from rapid burial and slow hydrous alteration into silicified casts (permineralization). Lying underground in a wet, mineral-rich environment, picking up hues of red and yellow and gray. Calmly, coolly, entirely without incident. A history about as far removed as possible from the sample that arrived in the mail over the weekend. Migmatites (from the Latin migma for mixture) are the product of intense heat and pressure that result both high-grade metamorphism and partial melting. Check out the Georneys post M is for Migmatite for fantastic coverage of all things migmatite.
What’s missing from this story is the provenance (origin) of this fantastic rock. Not all migmatites look the same – some lack the leucosomes (light bands) seen here, and they are not all black-and-white – and my hope is that this sample is from the Pacific Northwest… maybe someone out there knows where. For the past three decades it was hanging out east of the Cascades in Central Washington, which is a good place to start. The crustal accumulation and volcanic history of the Pacific Northwest is a prime migmatite-forming environment. I’ve found references to the Okanogan dome/highlands and the Skagit migmatite as starting points, but detailed online photographic records are somewhat lacking. Now I reach out to the ether: Have you seen this migmatite?